staging: octeon: Fix build failure due to typo.

drivers/staging/octeon/ethernet-tx.c:287:23: error: implicit declaration of function 'skb_drag_size'; did you mean 'skb_frag_size'? [-Werror=implicit-function-declaration]

From kernelci report:

	https://kernelci.org/build/id/5d3943f859b514103f688918/logs/



Fixes: 92493a2f ("Build fixes for skb_frag_size conversion")
Signed-off-by: David S. Miller <davem@davemloft.net>